But how long will the second honeymoon last for President Obama and the Democrats? Chances are, it won't be long until the left's hopes are dashed. Here are four possible letdowns on the horizon.

1. Entitlements: Obama has talked very tough on tax rates for income over $250,000, but you don't hear nearly as much strong rhetoric from the White House about progressives' other ironclad fiscal-cliff demand: protecting Social Security and Medicare. Back in 2011, during Obama and House Speaker John Boehner's failed attempt at a big deal on the debt ceiling, Obama was theoretically open to the kinds of changes Senate Minority Leader Mitch McConnell is now advocating, such as raising the eligibility age for Medicare from 65 to 67 and increasing premiums for recipients with higher incomes.

If a fiscal-cliff deal gets made, chances are it'll include some concessions along those lines. Obama's opening offer to Republicans included $400 billion in deficit reduction from unspecified changes to entitlements. Suzy Khimm has a helpful rundown of what entitlement changes Democrats have signaled a willingness to consider.

Thus far, defenders of Social Security have been gratified to hear White House Press Secretary Jay Carney say changes to the program are off limits in the negotiations, and even the $400 billion in the president's offer isn't too alarming -- that savings could come from reducing payments to pharmaceutical companies rather than changing eligibility or benefits, says Jeff Hauser, spokesman for the AFL-CIO, which has been campaigning hard against entitlement-benefit cuts. "Our red lines continue to be clear: December 31 should be the last day of the failed Bush tax cuts for the richest 2 percent, and there is no need for middle-class beneficiaries of Social Security, Medicare and Medicaid to contribute to deficit reduction for a deficit they did nothing to cause," Hauser said. But while the group would loudly protest any proposed cuts to benefits, he said, "we are open to improvements in the cost-effectiveness of our health-care system."

2. Elizabeth Warren: The liberal rock-star senator-elect from Massachusetts has the hopes of a movement riding on her shoulders as she prepares to move into the Capitol. Progressives seem to think she'll immediately become a major force in Washington, a triumphant, one-woman populist crusade that will quickly bring the big banks and corporations to their knees. Warren has already reportedly been promised a spot on the Senate Banking Committee, a gesture of respect on the part of Senate leaders for her cachet with the base, despite heavy pressure from Wall Street to keep her off.

But if Warren wants to be a player, she'll shun the limelight and put her nose to the grindstone, Senate veterans say. And those expecting her to shake up the institution are sure to be disappointed. "I understand my friends on the let have some high hopes that senator-elect Elizabeth Warren is going to come in and immediately change the place," said Jim Manley, a former longtime aide to Majority Leader Harry Reid and self-described institutionalist. "The fact is, that's not going to happen. The fact is, the Senate is much larger than any one individual. ... A freshman on the banking committee is on the end of the dais, the last one to speak, the last to question the witnesses. Star power is great, but the only way to affect change in the Senate is studying briefing books, attending hearings, getting to know your colleagues and carefully choosing your spots."

3. Filibuster reform: After years resisting the idea, Reid now says he supports making changes to the Senate rules, which have increasingly been used as a tool of universal obstruction by the minority. (Read David Graham's excellent primer on filibuster reform here.) This has the left, for whom ending the filibuster is a cherished priority, very excited. But "ending the filibuster" is a pipe dream -- any changes that do get made are likely to be minor; the leading candidate is ending procedural filibusters on "motions to proceed," while continuing to allow them on other votes. And while Reid is clearly on board with changing the rules in theory, there's another big question: Would he go as far as to try to change the rules with a simple majority vote on the first day of the Senate session in January, a constitutionally questionable step opponents have called the "nuclear option"? Or is he only willing to try to get it done with 67 votes under normal circumstances? Getting 67 votes for anything controversial in the Senate is a heavy lift.

4. Climate legislation: The devastation wrought by Hurricane Sandy inspired a fresh wave of interest in efforts to combat climate change. Notably, it brought New York Mayor Mike Bloomberg off the fence in the presidential race, inspiring him to endorse Obama just a few days before the election. The buzz around the issue, which was last seen dying in the Senate in 2010, was enough to get Obama asked about it in his post-election press conference.

But Obama's answer was rather noncommittal, and while the ensuing weeks have seen a parade of Republicans rushing to support immigration reform, there have been no notable defections when it comes to environmental legislation. Environmentalists say they've seen no indication the political calculus has changed since the election. "Ultimately, we're going to need climate legislation," said David Doniger, the policy director for the climate and clean air program at the National Resources Defense Council. "But that's not going to happen in this Congress, in our judgment, given the approach the Republican House in particular is taking" -- that is, trying repeatedly to strip the Environmental Protection Agency of its regulatory authority.

Instead, environmental activists are focused on getting more out of the EPA. This week, the NRDC issued a report and called on the agency to use the Clean Air Act to reduce carbon emissions from power plants. The group considers Obama's move to increase vehicle fuel-economy standards the biggest environmental accomplishment of his first term. "Environmental organizations are putting our emphasis on what President Obama and his agencies can do using laws already on the books," Doniger said. For those looking for action on climate change, that's probably the most realistic stance.